Underground Gibraltar

The naturalists of the eighteenth century, and certainly those of the nineteenth, noticed the fossil-bearing red strata in some parts of the Rock of Gibraltar. These fossil-rich Breccias were particularly noticeable in the area of Rosia Bay and much of this was removed during the eighteenth century as the cliffs were scarped to prevent amphibious assaults from the Spaniards. Some of the material was collected or recorded and there are some remnants visible even today. The breccias of the Rock became famous in the scientific community of the eighteenth and nineteenth centuries and aided significantly in the strengthening of ideas of organic evolution. John White provides a first class description:

“Not far from hence, on the S.W. side of Rosia Bay, was discovered, in the year 1769, a huge mass of petrifications of a very singular kind. The workmen who were employed in scarping the face of the rock to render it less accessible, after having wrought, by mining, through about ten feet of solid limestone came to a vast congeries of bones, blended and consolidated together in a confused manner with limestone of various sorts, freestone, spars, selenites, stalactites and calcareous crystallizations and incrustations. This curious assemblage of animal and fossil substances incorporated together extended to the space of ten or twelve feet every way in front; vast quantities of it were blown off in the prosecution of the work, and much more remains in the body of the rock. All of it has the appearance of having been thus blended in a fluid state; the bones are so universally equally intermixed that the smallest fragment cannot be found but what has a proportion of bones, the inner surfaces and the very pores of which are frequently found incrusted with glittering concretions. These bones lie jumbled across each other in the utmost confusion, retaining their original texture and colour when separated from the calcareous substances wherewith they are cemented together. There is no room to imagine that any of these are human bones, they all seem, on examination, to be those of sheep, or goats, or both.”

These breccias were probably formed during the middle Pleistocene, judging from the fauna contained within, and therefore rather earlier than the deposits at sites such as Gorham’s Cave. The fauna includes an extinct species of Rabbit named Prolagus calpensis by Forsyth Major in 1905. The process of formation of the breccias was through flash flooding. Torrential rains would have flowed down gullies on the southern and south-western slopes of Gibraltar and carried with them masses of red soil, along with stones and the remains of animals. These breccias are rich, therefore, in land snails and in mammal fossils and give us an idea of the fauna of this ancient Rock. As the material flowed towards the sea it followed lines of least resistance, often filtering into fissures and cracks within the Rock and clogging them up with sediment. The next set of rains would push more material downstream and move the earlier sediments along. Drier periods would have permitted the settling of this material which, in time, became compacted and hardened to form the breccia.